1. Field of the Invention
The present invention relates to an optical pickup device capable of performing at least one of reproduction, recording and erasing of information with respect to an optical disc (optical information recording medium) represented by a compact disc (referred to as CD) and digital versatile disc (referred to as DVD), and, more particularly, relates to an optical pickup device which controls an output of a laser beam using a front monitor light-receiving element.
2. Description of the Related Art
FIG. 8 is a schematic configuration diagram of a conventional optical pickup device using light receiving and emitting integral-type elements 1a and 1b in which a light-emitting element and a light-receiving element are integrated. In this optical pickup device, a laser beam emitted from each of the light receiving and emitting integral-type elements 1a and 1b and having an approximately horizontal optical axis 100 is transmitted or reflected by a beam splitter 12. The transmitted or reflected laser beam is collimated to a parallel beam by a collimator lens 13 and then, reflected by a raising mirror 14 approximately at a right angle to the upper portion where an objective lens 15 is disposed. The reflected laser beam is transmitted through the objective lens 15 and converged on a recording surface of a substantially horizontally disposed optical disc 16.
The converged laser beam is reflected on the recording surface of the optical disc 16 and entered to each of the light receiving and emitting integral-type elements 1a and 1b via a reverse path to a forward path. This laser beam is then guided to a light-receiving surface by a diffraction grating (not shown in the drawing) provided on each of the light receiving and emitting integral-type elements 1a and 1b. Then, an electric signal is detected according to the amount of the laser beam received by the light-receiving surface.
In such an optical pickup device, a part of the laser beam emitted from each of the light receiving and emitting integral-type elements 1a and 1b is entered to a front monitor light-receiving element 17 disposed substantially horizontally (that is, substantially parallel to the optical axis 100) in the vicinity of the beam splitter 12.
As shown in a FIG. 9, the front monitor light-receiving element 17 includes a substrate portion 50, a light-receiving portion 51 fixed to a lower surface of the substrate portion 50, and a transparent resin mold portion 52 sealing the light-receiving portion 51 to retain at a fixed position; and a light-receiving surface (lower surface) of the light-receiving portion 51 is disposed so as to be substantially parallel to the optical axis 100. The front monitor light-receiving element 17 outputs an electrical signal according to the amount of the laser beam entered to the light-receiving surface of the light-receiving portion 51 and controls the output of the light receiving and emitting integral-type elements 1a and 1b with an automatic power control (APC) circuit so that the laser beam to be converged on the optical disc 16 becomes appropriate intensity.
Other prior technology related the optical pickup device is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 2001-52368, 2002-92929, HEI09(1997)-159830, and HEI11(1999)-273119.
The optical pickup device shown in FIG. 8 and FIG. 9 is configured so that the laser beam is received by the front monitor light-receiving element 17 in which the light-receiving surface is substantially parallel to the optical axis 100 of the laser beam to monitor the laser beam output. This provides a significant advantage in that thickness in a direction (Y-axis direction) substantially perpendicular to the optical axis 100 of the laser beam in the optical pickup device can be thinner, as compared with the case where the light-receiving surface is disposed substantially perpendicular to the optical axis.
In such a configured optical pickup device, however, the light-receiving surface of the light-receiving portion 51 of the front monitor light-receiving element 17 is disposed substantially parallel to the optical axis 100 and the amount of laser beam entered to the front monitor light-receiving element 17 is not sufficient; and therefore, a control gain of the APC circuit needs to be increased. Whereas, when the control gain is increased, the responsiveness of the APC circuit is reduced. Consequently, the optical pickup device cannot correspond to high speed reproducing and recording information from and to the optical disc represented by a CD and DVD, and the optical pickup device may possibly be considerably degraded.
Furthermore, in such a configured optical pickup device, an incident angle θ of the laser beam to the front monitor light-receiving element 17 becomes large, as shown in FIG. 8. That is, output intensity distribution from a semiconductor laser is ellipse shape having a long axis in the Y-axis direction, a part of which is received by the front monitor light-receiving element 17; and the incident angle θ becomes large. Then, if an oscillation wavelength of the laser beam is changed, reflectivity on the surface of the front monitor light-receiving element 17 considerably fluctuates even if the incident angle θ is constant. Material of the front monitor light-receiving element 17 is semiconductor and wavelength dependence property of index of refraction is large. Therefore, there is a problem in that the output of the front monitor light-receiving element 17 changes even if the laser beam output is constant.
More specific explanation is as follows.
First, as well known, the relationship between the incident angle and the reflectivity is that the incident angle θ rapidly increases when exceeding approximately the so-called Brewster angle θB, whereθB=Tan−1n 
(n is a relative index of refraction of a transmission side medium to an incident side medium; here, n is an index of refraction of the front monitor light-receiving element 17; for example, θB becomes 76 degrees when n is 4). That is, incident angle dependence property of the reflectivity becomes precipitous on reaching the Brewster angle θB.
Meanwhile, when the index of refraction changes, the Brewster angle θB changes and therefore dependence property of the incident angle (θ) of the reflectivity (R) shown in FIG. 10 changes from a curve 1 to a curve 3 via a curve 2. As a result, even if the incident angle θ is constant, the reflectivity at the surface of the front monitor light-receiving element 17 fluctuates. Then, the incident angle (θ) dependence property of the reflectivity (R) becomes remarkable at such a large angle that the incident angle θ exceeds the Brewster angle θB.
Then, in the case of a semiconductor laser, the oscillation wavelength changes largely by an ambient temperature or temperature change due to self-heating and therefore fluctuation of the aforementioned reflectivity is a serious problem. Consequently, the laser beam needs to be entered to the front monitor light-receiving element 17 as vertically as possible without increasing the size of the device.